The arguments set forth by Mr.Lewis and Mr. Iriarte that the Orsova ballista was an inswinger, struck me as the most logical explanation for the unique geometry inherent in the original artifacts.  If we examine all of the ways these components can be fitted together with the following mock ups, the validity of their hypothesis becomes readily apparent.  Because the tangs on the arched strut are of different lengths, inserting them into the loops on the field frames limits the number of arrangements possible to the following three cases.  (I don’t include the fourth possible case of “curved stanchions facing out on the aft side of frame”  as even a remote possibility because the curved stanchion would interfere with any kind of sensible limb movement,  i.e. at rest limbs held nearly parallel to stock)

Case One. Curved stanchion facing inwards and on the aft side of the frame

In the first photo we might suspect that the curved stanchion allows the limbs to be retracted to an extreme degree to increase the length of the power stroke. However, the more the limb and string are brought into a straight line with one another the less leverage is available to move the limb tip back further. This is the commonly known phenomenon of “stacking” that the archer finds when shooting poorly designed short bows. Things go fine through mid draw and then at some point the increasingly unfavorable string angle puts the draw force into an exponential curve.   A classic case of diminishing returns.  Moreover, as seen in the second photo, the straight stanchion is not in the correct position to act as a limb arrestor, nor would it be substantial enough to act as one even if it was. We can forget about the commonly held view that the bowstring would act to stop the limb moving forward at the end of its stroke much like it does on the simple hand bow.  The 12,000 lb. test dacron yacht braid used on the Gallwey bowstring was easily able to handle its 4000 lb maximum draw weight, but it (or the eye loops spliced into its ends) soon stretched out when subjected to the full snap of the limbs going forward.  The sudden jerk on the bowstring coming from a powerful ballista is going to quickly stretch any string to the point that the limbs will keep rotating forward until they hit something substantial.   Clearly, some type of shock absorbing pad is needed to prevent damage to the limb and to the stanchion it smashes into.   For these reasons, the Case One arrangement seems pretty dubious.

Case Two. Curved stanchion facing inwards and on the forward side of the frame.

I originally spent a fair amount of time thinking that an argument could be made for having the curved stanchions in this position as a way to pre-load the spring bundles by using the limbs themselves. If the limbs were allowed to rotate forward into the pockets of the curved stanchions and then the washers were locked into place to the field frames, retracting the limbs to slip on the bowstring would naturally twist up the spring bundles to pre-load them. What a lot of nonsense that turned out to be.  Again there is no limb arrestor with this configuration,  which pretty well kills the idea right there.  Also, if we measure the length of draw available before the limbs rearward movement is stopped by the straight stanchions, it is an unimpressive 34”.  For a machine of this size, that is way wimpy.

Case Three. Curved stanchions are facing outwards and on the forward side of the frame.

In this position,  the heavy duty curved stanchions are in the perfect position to act as limb arrestors. Length of draw goes to a whopping 63”.  Also, the great distance apart between the spring bundles (compared to a conventional outswinger) now makes sense as it provides the room necessary for the limbs to travel through their arc of movement without hitting the stock.  Comparing this configuration to the other two cases, there doesn’t seem to be much of an argument in their favor.  The Orsova ballista was clearly intended to be an inswinger.  To eyes that are accustomed to seeing the limbs and bowstring in the conventional position of an outswinger, the above inswinger looks awkward and can violate more traditional sensibilities.  At least that is how I felt when I first looked at it.  However, I am also a firm believer in the notion that form must follow function.  After actually handling the reproductions of these artifacts and fitting them together in every possible configuration, this radical interpretation does seem to be inescapable.

Was the inswinger, like so many of the innovations used by the Romans, the invention of some unknown Greek engineer?  Or was it something truly new and radical that the Romans had come up with themselves?  Probably that is something we will never know.  I can recall reading that towards the end of the Roman Empire there were certain politicians who spoke of a whole new class of secret weapons that would be able to save the day.  Was the Orsova ballista one of the designs they were talking about?  Very likely I should think.  However, like so many promises of that kind,  larger events overtook any tactical advantage the new design may have rendered.

One Response to “The case for an inswinger”

  1. "Warhammer1" says:

    Ahhhhh. Your language and descriptions are becoming increasing more elegant, as your explanation(s)in this case of the inswinger and “stacking” that I tried to pass on in an earlier post (my first one here?).

    I arrived at the inswinging design myself after a good bit of mental exercise, and its superiour velocity potential proved itself through the prototyping processes. Outswinging designs CANNOT compete with the inswingers as you so elequently stated.

    Todays “new” crossbow designs have only gone as far as a “reverse paralell limb” which was only recently patented and employed ( ) and two other companies Armscross based in Germany, and Horton (the Recon and Vision models) Crossbows which is now made in China (yuck). Fortunately they cannot patent the inswinging design as it already has been. Eventually though crossbow companies may try some original thinking, only to discover the tech. is a thousand years old…

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